1. Field of the Invention
The present invention relates to a color separator circuit for separating colors to reproduce chrominance signals from image signals and to a chrominance signal processing device provided with such a color separator circuit. The present invention relates particularly to a color separator circuit that performs color separation on image signals output from a solid-state image-sensing device and to a chrominance signal processing device provided with such a color separator circuit.
2. Description of the Prior Art
When chrominance signals are produced from image signals output from a solid-state image-sensing device, such as a single-panel color CCD (charge-coupled device) or a two-panel color CCD, that is provided with a plurality of types of color filters, from the image signals obtained for each type of color filter, signals that are supposed to be obtained for the other types of color filter than the one provided for the pixels that are currently yielding image signals are produced by interpolating neighboring image signals. Then, by using the signals thus produced by interpolation and the image signals output from the CCD, color separation is performed, then primary color signals, i.e. R (red), G (green), and B (blue) signals are produced, then color difference signals Rxe2x88x92Y and Bxe2x88x92Y are produced, and eventually chrominance signals are produced
FIG. 6 shows a conventional chrominance signal processing device that produces chrominance signals from image signals output from a CCD as described above. In the chrominance signal processing device shown in FIG. 6, when image signals are fed in from a CCD, they are fed to a line memory 51 and to an adder circuit 53. The image signals output from the line memory 51 are fed to a line memory 52 and to a color separator circuit 55, and the image signals output from the line memory 52 are fed to the adder circuit 53. In this way, image signals from one row after another are stored in the line memory 51 and then in the line memory 52. The adder circuit 53 is fed with the image signals of the first row from the line memory 52 and the image signals of the third row directly from the CCD, and the image signals added together by the adder circuit 53 then have their signal levels multiplied by xc2xd by a multiplier circuit 54 so that the image signals of the first and third rows are averaged. Then, the image signals output from the multiplier circuit 54 and the image signals of the second row output from the line memory 51 are fed to the color separator circuit 55.
The color separator circuit 55 produces, for each image signal, three signals, namely a luminance signal YL and two color separation signals Cr and Cb, and feeds them to an RGB matrix circuit 56. From the luminance signal YL and the color separation signals Cr and Cb, the RGB matrix circuit 56 produces primary color signals, namely R, G, and B signals, from which a color difference matrix circuit 57 then produces color difference signals Rxe2x88x92Y and Bxe2x88x92Y. These color difference signals Rxe2x88x92Y and Bxe2x88x92Y are fed to a color encoder 58, which then produces and outputs chrominance signals.
The color separator circuit 55 is provided with color separation filters 59 and 60 for interpolating or correcting the image signals fed from the line memory 51 in the horizontal direction, color separation filters 61 and 62 for interpolating or correcting the image signals fed from the adder circuit 53 in the horizontal direction, an adder circuit 63 for adding together the outputs from the color separation filters 59 and 60, a subtractor circuit 64 for calculating the difference between the outputs from the color separation filters 59 and 60, an adder circuit 65 for adding together the outputs from the color separation filters 61 and 62, a subtractor circuit 66 for calculating the difference between the outputs from the color separation filters 61 and 62, and an adder circuit 67 for adding together the outputs of the adder circuits 63 and 65.
Suppose that the chrominance signal processing device configured as described above is fed with image signals output from a CCD provided with four types of color filters, namely M (magenta), G (green), Y (yellow), and C (cyan) color filters, as shown at (a) in FIG. 3. As shown at (a) in FIG. 3, the CCD has two types of columns of color filters arranged alternately, specifically columns in which color filters are arranged in the order of M, Y, G, and Y and columns in which color filters are arranged in the order of G, C, M, and C. Moreover, the CCD outputs image signals obtained from two adjacent rows in combination. Specifically, as shown at (b) in FIG. 3, for every two rows, the CCD outputs image signals M+Y, G+C, G+Y, and M+C.
Let these image signals be expressed also as C1=M+Y, C2=G+C, C3=G+Y, and C4=M+C, respectively. Where image signals are output in this way, the colors M, C, and Y are expressed, in terms of primary colors R (red), G (green), and B (blue), as M=R+B, C=G+B, and Y=R+G, respectively. Hence, the image signals C1, C2, C3, and C4 are expressed, in terms of primary colors R, G, and B, as C1=2R+G+B, C2=2G+B, C3=2G+R, and C4=2B+G+R, respectively.
When image signals have been fed in in this way, for example, the chrominance signals for pixels that yield the image signals C1 are produced in the following manner. First, the image signals C1 and C2 stored in the line memory 52 are fed to the color separation filters 59 and 60. Thus, the color separation filters 59 and 60 output the interpolated or corrected image signals C1 and C2. On the other hand, the image signals C3 and C4 fed directly from the CCD and the image signals C3 and C4 fed from the line memory 51 are averaged by the adder circuit 53 and the multiplier circuit 54, and are then fed to the color separation filters 61 and 62. Thus, the color separation filters 61 and 62 output the interpolated or corrected image signals C3 and C4.
When the image signals C2 to C4 for the pixels that yield the image signals C1 have been calculated plausibly in this way, the adder circuit 63 adds together the image signals C1 and C2, and the subtractor circuit 64 calculates the differences between the image signals C1 and C2. Simultaneously, the adder circuit 65 adds together the image signals C3 and C4, and the subtractor circuit 66 calculates the differences between the image signals C3 and C4. Then the outputs from the adder circuits 63 and 65 are added together by the adder circuit 67 to produce luminance signals YL, and the subtractor circuits 64 and 66 output color separation signals Cr and Cb, respectively. When the luminance signals YL and the color separation signals Cr and Cb have been produced in this way, the RGB matrix circuit 56 produces primary color signals, then the color difference matrix circuit 57 produces color difference signals, and then the color encoder 58 produces and outputs chrominance signals.
In the chrominance signal processing device shown in FIG. 6, the color separation filters 59 to 62 provided in the color separator circuit 55 thereof are each designed as a filter that performs calculation on three horizontally adjacent image signals, i.e. a target image signal and the image signals immediately preceding and succeeding it, with the color separation filters 59 and 61 given filtering characteristics (0, 2, 0) and the color separation filters 60 and 62 given filtering characteristics (1, 0, 1).
Suppose that a color separation filter is given filtering characteristics (a, b, c), that the image signal from the pixel for which the chrominance signals are currently being calculated has a signal level xe2x80x9ccbxe2x80x9d, and that the image signals output immediately before and after this image signal having the signal level xe2x80x9ccbxe2x80x9d have signal levels xe2x80x9ccaxe2x80x9d and xe2x80x9cccxe2x80x9d, respectively. Then, the color separation filter having these filtering characteristics produces and outputs an image signal having a signal level xe2x80x9caxc3x97ca+bxc3x97cb+cxc3x97ccxe2x80x9d. In the filtering characteristics (a, b, c) of a color separation filter, the components, here xe2x80x9cbxe2x80x9d, and xe2x80x9ccxe2x80x9d, are called xe2x80x9ctapsxe2x80x9d, of which the number represents the number of image signals that the color separation filter uses to produce a signal.
Thus, with the color filters 59 to 62 having the filtering characteristics (0, 2, 0) and (1, 0, 1) as described above, for example, when image signals C1 and C2 of which the signal levels vary as shown in FIG. 7A are fed to the color separation filters 59 and 60, they are output with their signal levels interpolated as shown in FIGS. 7B and 7C. Accordingly, when an edge is encountered at a position A between the fourth and fifth columns of pixels as shown in FIG. 7A, the image signals C1 and C2 immediately preceding and succeeding the position A are averaged and thereby interpolated by the image signals C1 and C2 preceding and succeeding them.
The image signals C1 and C2 thus interpolated and output from the color separation filters 59 and 60 are fed to the subtractor circuit 64, which then subtracts the image signals C1 from the image signals C2 and thereby produces color separation signals Cr. Here, whereas ideally the signal levels of the color separation signals Cr should remain 0 all the time as shown in FIGS. 7E, 7F, and 7G, in reality they become greater than 0 around the edge encountered at the position A as shown in FIGS. 7B, 7C, and 7D, and thus contain certain non-zero components. This causes the edge to appear falsely colored.
On the other hand, with the color separation filters 59 to 62 given a larger number of taps, when image signals C1 and C2 as shown in FIG. 7A are fed thereto, they are interpolated and output as shown in FIGS. 7H and 7I. Here, the levels of the image signals C1 and C2 that are actually output are themselves corrected by the image signals preceding and succeeding them. Thus, the levels of the image signals C1 and C2 that are actually output themselves vary, and as a result the signal levels of the color separation signals Cr are closer to the ideal value around the edge as shown in FIG. 7J than is shown in FIG. 7D. This helps alleviate the false coloring of the edge. However, the color separation signals thus obtained have signal levels deviated from the ideal value on the whole, and thus the portion of the image corresponding thereto is falsely colored on the whole.
As described above, in interpolating image signals in the horizontal direction, using color separation filters with a small number of taps causes the signal levels of the color separation signals to deviate greatly from the ideal value around an edge, and using color separation filters with a large number of taps causes the signal levels of the color separation signals to deviate from the ideal value on the whole. Thus, in either case, false coloring occurs.
On the other hand, in the vertical direction, the image signals fed directly from the CCD and the image signals fed from the line memory 52 are averaged and thereby linearly interpolated by the adder circuit 53 and the multiplier circuit 54. As a result, when the subtractor circuit 64 outputs color separation signals Cr, the subtractor circuit 66 outputs linearly interpolated color separation signals Cb. Now, suppose that, as shown in FIG. 8A, an edge is encountered at a position B between the second and third rows. Then, the subtractor circuit 64 outputs color separation signals Cr for the first and third rows and color separation signals Cb for the second and fourth rows, respectively having signal levels as shown in FIGS. 8B and 8C.
Here, as shown in FIGS. 8D and 8E, the signal level of the color separation signal Cr of the second row output from the subtractor circuit 66 is equal to the value obtained by linearly interpolating the signal levels of the color separation signals Cr of the first and third rows, and the signal level of the color separation signal Cb of the third row output from the subtractor circuit 66 is equal to the value obtained by linearly interpolating the signal levels of the color separation signals Cb of the second and fourth rows. Since an edge is encountered at the position B now, the ideal values of the signal levels of the color separation signals Cr and Cb are as shown in FIGS. 8F and 8G. As will be clear from comparison between what is shown in FIGS. 8D and 8E and what is shown in FIGS. 8F and 8G, the color separation signal Cr of the second row and the color separation signal Cb of the third row have signal levels either lower or higher than their ideal values. Thus, when linear interpolation is performed in this way, false coloring occurs in the vertical direction also.
An object of the present invention is to provide a color separator circuit and a chrominance signal processing device that alleviate false coloring when image signals for a plurality of types of color filters are obtained by interpolation and chrominance signals are produced from such image signals.
To achieve the above object, according to one aspect of the present invention, a color separator circuit for performing color separation on image signals fed thereto from a solid-state image-sensing device having pixels provided with a plurality of types of color filters is provided with: a contour detector for detecting the contour of a subject sensed by the solid-state image-sensing device by recognizing variations in the signals levels of the image signals fed from the solid-state image-sensing device; a first color separation filter for correcting the image signals fed from the solid-state image-sensing device by correcting each image signal based on a plurality of preceding and succeeding image signals; a second color separation filter for correcting the image signals fed from the solid-state image-sensing device by correcting each image signal based on a plurality of preceding and succeeding image signals; and a selector for selecting the second color separation filter when correcting image signals that represent a portion of an image that corresponds to the contour detected by the contour detector and selecting the first color separation filter when correcting image signals that represent a portion of the image other than the portion corresponding to the contour. Here, the first color separation filter uses a smaller number of image signals to correct an image signal than the second color separation filter.
According to another aspect of the present invention, in a chrominance signal processing device that produces chrominance signals based on signals output from a color separator circuit provided therein, the color separator circuit is provided with: a contour detector for detecting the contour of a subject sensed by a solid-state image-sensing device having pixels provided with a plurality of types of color filters by recognizing variations in the signals levels of image signals fed from the solid-state image-sensing device to the contour detector; a first color separation filter for correcting the image signals fed from the solid-state image-sensing device by correcting each image signal based on a plurality of preceding and succeeding image signals; a second color separation filter for correcting image signals fed from the solid-state image-sensing device by correcting each image signal based on a plurality of preceding and succeeding image signals; and a selector for selecting the second color separation filter when correcting image signals that represent a portion of an image that corresponds to the contour detected by the contour detector and selecting the first color separation filter when correcting image signals that represent a portion of the image other than the portion corresponding to the contour. Here, the first color separation filter uses a smaller number of image signals to correct an image signal than the second color separation filter.